Exhaust-gas treatment devices in which a liquid additive is required are required for example for the SCR process. In the SCR (Selective Catalytic Reduction) process, nitrogen oxide compounds in the exhaust gas of an internal combustion engine are reduced, with the aid of ammonia, to form non-hazardous substances such as nitrogen, water and CO2. The ammonia for the SCR process is normally produced from a liquid ammonia precursor solution which is stored in a tank and which may be delivered from the tank to the exhaust-gas treatment device by means of the described pump. The liquid ammonia precursor solution is also referred to as liquid additive, as reducing agent or as reducing agent precursor. The liquid is preferably urea-water solution. What is particularly preferable is a urea-water solution with a urea content of about 32.5%, which is available under the trade name AdBlue®. The liquid is converted to ammonia outside the exhaust gas (in a reactor provided for the purpose) and/or in the exhaust gas (in the exhaust-gas treatment device).
A problem in the case of pumps for the delivery of such liquids is that these liquids may freeze at low temperatures. The described 32.5% urea-water solution, for example, freezes at −11° C. In motor vehicles, such low temperatures may arise in particular during long standstill phases in winter. Liquid expands as it freezes. Either the pump must be designed such that it is not damaged by freezing liquid, or an evacuation of the pump must take place if low temperatures are encountered. In order that a pump may be evacuated, it is generally necessary that the pump also generates air flow.
In particular in the case of a urea-water solution as liquid, it is also a problem that, for example, crystalline urea deposits may form or gaseous ammonia may be excreted, which may influence the operation of the pump. Crystalline urea deposits may influence components of the pump through friction, and cause material abrasion on the components of the pump.
Pumps for delivering such liquids to an exhaust-gas treatment device should preferably also be capable of delivering the liquid to the exhaust-gas treatment device in as precisely dosed a fashion as possible. This makes it possible to dispense with additional measures for dosing the liquid. Additional measures for dosing are for example dosing valves by means of which the dosing may be performed in a time-controlled manner (by way of the opening time of the dosing valve). Dosing by means of the pump is often significantly more accurate than dosing valves of this type.
The pump should furthermore have the highest possible mechanical efficiency, such that the least possible energy is required for the delivery and dosing of the liquid and, at the same time, the least possible heating of the pump occurs.
Documents U.S. Pat. Nos. 2,544,628, 3,408,947, DE 2 853 916 and DE 38 15 252 A1 disclose a pump type also referred to as an orbital pump. This type of pump is relatively resistant to a volume expansion of a liquid when the latter freezes.
Secondly, this type of pump may also be operated in a reverse delivery direction, such that an evacuation of a delivery module is possible in a technically simple manner. It is however necessary for this type of pump to be adapted to the requirements imposed by the SCR process. It is sought in particular to achieve an improvement of this type of pump with regard to dosing accuracy. This type of pump is problematic in particular with regard to efficiency and dosing accuracy because, a very large amount of mechanical energy is required to deform the diaphragm element, and the deformability of the diaphragm element may lead to dosing inaccuracy.